Droplet ejection devices are used for a variety of purposes, most commonly for printing images on various media. They are often referred to as ink jets or ink jet printers. Drop-on-demand droplet ejection devices are used in many applications because of their flexibility and economy. Drop-on-demand devices eject one or more droplets in response to a specific signal, usually an electrical waveform that may include a single pulse or multiple pulses. Different portions of a multi-pulse waveform can be selectively activated to produce the droplets.
Droplet ejection devices typically include a fluid path from a fluid supply to a nozzle path. The nozzle path terminates in a nozzle opening from which drops are ejected. Droplet ejection is controlled by pressurizing fluid in the fluid path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. The actuator changes geometry or bends in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. Deposition accuracy is influenced by a number of factors, including the volume and velocity uniformity of drops ejected by the nozzles in the head and among multiple heads in a device. The droplet size and droplet velocity uniformity are in turn influenced by factors such as the dimensional uniformity of the ink paths, acoustic interference effects, contamination in the ink flow paths, and the actuation uniformity of the actuators.
Each ink jet has a natural frequency which is related to the inverse of the period of a sound wave propagating through the length of the ejector (or jet). The jet natural frequency can affect many aspects of jet performance. For example, the jet natural frequency typically affects the frequency response of the printhead. Typically, the jet velocity remains near a target velocity for a range of frequencies from substantially less than the natural frequency up to about 25% of the natural frequency of the jet. As the frequency increases beyond this range, the jet velocity begins to vary by increasing amounts. This variation is caused, in part, by residual pressures and flows from the previous drive pulse(s). These pressures and flows interact with the current drive pulse and can cause either constructive or destructive interference, which leads to the droplet firing either faster or slower than it would otherwise fire. Constructive interference increases the effective amplitude of a drive pulse, increasing droplet velocity. Conversely, destructive interference decreases the effective amplitude of a drive pulse, thereby decreasing droplet velocity.
FIG. 1 illustrates a waveform of an ink jet according to a prior approach. The ink jet includes an actuator that is flexed or fired when voltage is applied. This waveform fires a droplet by first creating an initial negative pressure (fill) and then holds the actuator in this position as a pressure wave propagates through a pumping chamber. Upon the reflection of pressure wave at the end of the chamber, the actuator applies a positive pressure (fire) in phase with the pressure wave's reflection. Subsequent drive pulses may constructively or destructively interfere with previous pressure waves leading to variations in droplet velocity.
The volume of a single ink droplet ejected by a jet in response to a multi-pulse waveform increases with each subsequent pulse. The accumulation and ejection of ink from the nozzle in response to a multi-pulse waveform is illustrated in FIG. 2. Prior to an initial pulse, ink within an ink jet terminates at a meniscus which is curved back slightly (due to internal pressure) from an orifice of a nozzle. Following the ejection of a droplet, the ink within an ink jet should again terminate at the meniscus within a nozzle. The waveform in FIG. 1 produces a meniscus bounce as illustrated in FIG. 2 based on a portion of an ink droplet not breaking off and being ejected. Rather, this portion oscillates and stays attached to ink within the nozzle. This can lead to more variation in ejected droplet volume and adversely affect subsequent droplet ejection.